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Buy the Canon EF 20-35mm f/3.5-4.5 USM

(From Canon lens literature) A highly portable lens with high optical performance and a light weight of 12 oz. (340g). A very practical ultra-wide-angle zoom. The large front lens group minimizes peripheral darkening, and the flare-blocking diaphragm minimizes flare. Also, with lens group 2 being the zoom group, distortion is corrected.

This is a very useful lens wide angle lens for a full frame camera if you're on a budget or need to pack light. If you're looking for ultimate image quality, click elsewhere. If you want a cheap 20mm lens that is reasonably well built and gives good image quality at f/5.6 or f/8, this may be what you're looking for.

On a APS-C camera, there are plenty of better options than this. This lens is all about getting you into UWA territory on a full frame camera without breaking the bank.

I bought this lens with my EOS 10s. That makes it old and not used much anymore. Most of the lens short comings can be helped in CS2. It has served me well however the I've purchased the 16-35L recently and the comparison is drastic.
The 16-35L is also an additional $1000, so from that point I'll keep this lens as an old standby, or just an old friend.

But the IQ never could convince me. As a wideangle I used it for landscapes and cityscapes, and found the contrast and colors to be lacking. Also straight lines in the middle of the frame got distorted.

I also felt that the 35mm was too short for me, as quick portraits turned out strangely distorted.

So I switched to the Sigma 18-50/2,8 which is a much better lens IMHO (apart from some AF issues).

This is a practical lens, very well conceived: it provides a flare cutting diaphragm, both control rings operates smoothly, the front lens doesn't rotate, and the physical length remains constant while zooming. USM provides really fast and nearly silent autofocus operations.
Despite its low weight, this lens is well built: it isn't an L class lens, but my specimen performs flawlessly after four years of intensive usage. The optical quality is quite good, but not stellar: at the wide end distortion may be visible, and in some situations borders are quite soft. Flare can sometimes be an issue, but only in rather extreme situations; lens hood is recommended, as usual. In flare and especially in ghosting performances, this zoom outperforms every competitor in its price range.
All in all, this is an honest workhorse, recommended for full frame or 1,3x SLRs: on APS-C sensors, CA and resolution could be disappointing, and the zoom range wouldn't be so attractive.

This lens has an internal flare cutting, rectangular, plate which is visible looking into the front of the lens. It does a good job for a multi-element wide-angle zoom with flare rarely being a problem. The ring type USM is fast and quiet. The (expensive) lens hood is very large, makes the lens look huge and doesn't make much difference to flare.

There is no serrated area at the back of the lens to grip it when attaching/removing it from the body. My problem came about when I mounted it by holding the (fixed) front part of the lens. Over time the front started to come loose. After repair by Canon there was more barrel distortion than before at 20mm so I sold it.